Graphite is a crystalline form of carbon comprising atoms bonded in flat layered planes with weaker bonds between the planes. By treating particles of graphite, such as natural graphite flake, with an intercalant of, e.g., a solution of sulfuric and nitric acid., the crystal structure of the graphite reacts to form a compound of graphite and the intercalant. The treated particles of graphite are hereafter referred to as "particles of intercalated graphite". Upon exposure to high temperature, the particles of intercalated graphite expand in dimension as much as 80 or more times its original volume in an accordian-like fashion in the c-direction, i.e. in the direction perpendicular to the crystalline planes of the graphite.
Particles of intercalated graphite have many useful applications. The most common application is to exfoliate the intercalated graphite particles into vermicular like structures which are then compressed into sheets of flexible graphite for use in the manufacture of gaskets or as packing material. Particles of intercalated graphite may also be used in their unexpanded state to form a variety of products which take advantage of the high expansion characteristic of the particles of intercalated graphite when exposed to high temperature. One such example is for use in combination with polymer foams to form seat cushions and furniture upholstery in aircraft. Upon exposure to fire, the high temperature will cause the particles of intercalated graphite to exfoliate which minimizes or prevents the formation of toxic gases from the polymer foam and may, of itself, smother a fire. Despite its apparent advantages, commercial use has been limited due to the high concentration of titratable surface acids present on the particles of intercalated graphite. An average of about 4% surface acids by weight have been calculated as being present on commercially available particles of intercalated graphite. This is considered to be much too high in concentration for many of the above suggested uses. A high concentration of titratable surface acids is objectionable for the following reasons: (a) it gives off a disagreeable odor, (b) its volumetric expansion capability is reduced with age and (c) surface acids will interact chemically with polymer foam catalysts and interfere with the desired foaming action.
Considerable effort has been undertaken to minimize the concentration of surface acids present on particles of intercalated graphite with minimal success. Most of this effort was based on the premise that the concentration of surface acid could be controlled by proper washing of the intercalated flakes in water. In fact, many investigators have concluded that a relatively high concentration of surface acids is inherent to the process of intercalation and cannot be effectively eliminated without seriously impairing the volumetric expansion capability of the intercalated particles. It has been discovered in accordance with the present invention that particles of intercalated graphite may be formed with a very low concentration of titratable surface acid and a volumetric expansion of over 150 cc/g, independent of the method of washing and drying.